TF2 didn't even have microtransactions until 2010, and it both pretty much invented them and let you trade/sell them (albeit for steam bucks), so there are definitely worse games you could be throwing money at.
I bought the Orange Box the day it came out, and when I stopped playing TF2 a decade ago I sold all my items (which I mostly got from buying other games that I would have bought anyway), and I ended up with more steam money than I had ever actually spent on TF2 directly.
Making content for online is way less work and makes more money in the longrun. You don't get whales spending thousands of dollars on a single player DLC.